Epoxy resins have good mechanical properties, high resistance to heat and chemicals, and provide high bond strength, and hence, they are widely used as thermosetting resins for use in laminated sheets, adhesives or paints. Thin films have been conventionally prepared from synthetic resins such as epoxy resins by dispersing them in organic solvents, but with the current concern over pollution control and resources saving, active efforts are being made in various fields to develop water-dispersible resins.
Two methods for dispersing synthetic resins in water have been proposed; they are (1) the mechanical emulsification method wherein the resin is emulsified in a water-base medium in the presence of a surfactant, and (2) the hydrophilic group grafting method wherein a polymerizable hydrophilic vinyl compound is grafted onto the resin. The first method is currently used to make an aqueous dispersion of epoxy resins, but this method has the following disadvantages: (1) the epoxy resin is unstable and easily separates from water; (2) the epoxy resin is not uniformly dispersed in water, so that the subsequent curing reaction does not proceed evenly and a product of good performance is not obtained; (3) since the surfactant is used in a large amount, the thin film produced has low water resistance and bond strength, and furthermore, the residual surfactant dissolves out of the thin film into water. The second method is a rather recent technique (see Japanese Patent Public Disclosure No. 1228/78) which consists of producing an aqueous dispersion by neutralizing the hydrophilic groups in the graft molecule with alkali or acid. But this method also has the following defects: (1) the reaction requires a large amount of organic solvent, and the residual monomer is difficult to remove; (2) the grafting efficiency is so low that the ungrafted polymer easily separates from the grafted polymer; (3) since the neutralization step follows the grafting of hydrophilic groups, the epoxy resin is not uniformly dispersed in water and easily separates from water; (4) because of reason (3), the subsequent curing reaction does not proceed evenly and a product of good performance is not obtained.
Therefore, it will be of great benefit for the industry to establish a method for synthesizing a chemically stable water-dispersible epoxy resin composition that has high resistance to water and solvents, as well as good physical properties.